Whether
eggs in a single clutch will hatch simultaneously or
sequentially over an extended period of time is determined
by the onset of incubation. In many birds, including most
precocial species, incubation does not begin until the last
egg has been laid, resulting in all of the eggs hatching
within a few hours of each other. In contrast, many other
birds begin incubation prior to laying the last egg of the
clutch. This results in asynchronous hatchings separated by
anywhere from a few hours to several days, depending on how
soon incubation commences following the start of egg
laying. In altricial and
semialtricial species, asynchronous hatching gives the first
chicks to leave the egg a head start at vigorously begging
for food and successfully attracting parental attention. The
influential British ornithologist David Lack viewed the
evolution of asynchronous hatching as a parental strategy
for raising the largest number of offspring that food
resources will allow when the abundance of food for the
chicks cannot be predicted at the time that eggs are laid.
The matching of offspring number with food availability is
thus achieved by means of brood reduction: with asynchronous
hatching, the smallest chick or chicks do not garner "their
fair share" and will only survive in years of abundant
food. Elimination of the smallest
chicks usually occurs by starvation as result of competition
with their larger siblings for parental feeding, from overt
parental neglect, or psychological and physical intimidation
by their larger siblings. Brood reduction by means of
starvation routinely occurs in almost all of our gulls and
raptors, and is commonly seen in birds as diverse as
cormorants, herons, egrets, terns, Red-cockaded Woodpeckers,
Curve-billed Thrashers, some corvids, and Common Grackles.
Size disparity among chicks is exacerbated in gulls by the
laying of different-sized eggs in which egg weight typically
decreases with laying order. Eagles and boobies exhibit
"obligate siblicide," in which the larger chick invariably
kills its smaller sibling. For example, of more than 200
records of two-egg clutches followed in the Black Eagle of
southern Africa, only one record exists of both chicks
surviving to fledging. Obligate siblicide also occurs among
pelicans, owls, and cranes. In obligate siblicide, which
occurs even when food supplies are abundant, the second egg
serves as insurance against loss of the first egg from
infertility, predation, or damage, rather than as a means of
rearing two chicks. In the 25-40 years since
Lack's studies, information about many additional species
has accumulated, and hatching asynchrony is now thought to
be more common than synchronous hatching among altricial
birds. Ornithologists now think that asynchronous hatching
is not a strategy for achieving brood reduction in many
species. They have advanced other hypotheses to explain its
evolution. Sociobiologists Anne Clark and David Wilson argue
that high nest predation rates can encourage the evolution
of asynchronous hatching as a means of minimizing the total
amount of time that eggs and nestlings spend in the nest.
Asynchronous hatching also can be interpreted for some
insectivorous species as an adaptation to "speed up"
hatching so that at least some of the nestlings can
capitalize on rapidly (but unpredictably) peaking food
resources (such as outbreaks of forest
caterpillars). This "new view" of the
adaptive significance of hatching asynchrony helps to
explain the increasing egg weight often seen from first to
last egg laid among many asynchronously hatching passerines.
These weight differences represent differing proportions of
nutrients and energy invested in different eggs within
clutches, with the greatest investment going into the last
egg of the clutch. This pattern is clearly inconsistent with
the notion of facilitating brood reduction, and appears to
be a means of compensating for the delay in hatching and
development by providing the last chick with more resources
initially. SEE: Incubation
Time;
Precocial
and Altricial Young;
Eggs
and Their Evolution;
Variation
in Clutch Sizes;
Average
Clutch Size. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.